1. Field of the Invention
The present invention relates to the improvement of a power source for the developing bias to be impressed on the developing section of an electrostatic copying machine.
2. Description of the Prior Art
Electrostatic latent images formed on the electrophotographic photosensitive receptor are developed at the developing section and a bias voltage is impressed between the electrically conductive supporting member of the photosensitive receptor and the developing sleeve in order to prevent a fog, namely, a grey background to be caused.
A circuit shown in FIG. 1 or FIG. 2 has been employed so far as a circuit to impress the bias voltage. Regarding the circuit in FIG. 1, it has a drawback that output voltage setting requires elements for high-voltage and switching elements for high-voltage (QA, QB, QC, . . . ) in the amount of number of switching steps (A, B, C, . . . ) are required for high-voltage switching. A circuit to control the output variation in FIG. 2, that is, a circuit to control the pulse width for switching by means of a feedback loop has drawbacks that it is difficult to obtain steadily the switching of output voltage and variable actions in a broad range and further the circuit will be complicated and expensive because an error-amplifier and others are needed. Incidentally, aforesaid power supply for developing bias in the copying machine is the one wherein the voltage of about 500 V is impressed between said units and a current of about 10 .mu.A flows for example, and it is clear from the foregoing that the supplying power source has only to be a power source with constant voltage to be supplied to the load with a very few variation.